As petroleum-based materials escalate in price and environmental pressures increase, there is a growing need to responsibly utilize, to the greatest extent possible, all products from petrochemical processes, which includes the byproducts that are unavoidably formed along with the desired main products. Reaction byproducts are often mixtures that may be complex in composition, difficult to use directly, and difficult and/or costly to purify. They are frequently treated as materials of little or no value, for example being discarded or burned for fuel value.
It is known in the manufacture of adipic acid or caprolactam from cyclohexane that byproduct streams result because the chemical transformations do not proceed perfectly in 100% yield. These byproduct streams contain a variety of molecules having functionalities which include, among others, one or more alcohol, alkene, carboxylic acid, lactone, ester, and ketone groups, and combinations thereof. These byproduct streams are complex mixtures. It is known to use some byproduct streams for their fuel value. In such uses, there is little or no recognition or recovery of value for the functionality present in the byproduct stream. As a result, most of the byproduct stream from adipic acid manufacture remains underutilized.
Manufacture of adipic acid from cyclohexane generally involves two steps. First, cyclohexane is oxidized using air to a mixture of cyclohexanol (A) and cyclohexanone (K), the mixture being referred to as KA. Second, KA is oxidized using nitric acid to adipic acid.
A similar “cyclohexane oxidation” step is also performed in manufacture of caprolactam from cyclohexane. In the caprolactam manufacturing process, cyclohexanone is converted to its oxime, which is then caused to undergo molecular rearrangement to yield caprolactam. Caprolactam can then be polymerized to provide nylon-6.
In the known cyclohexane oxidation processes, cyclohexane is generally oxidized with oxygen or a gas containing oxygen, at low conversion, to produce an intermediate stream containing cyclohexanol (A), cyclohexanone (K), and cyclohexyl hydroperoxide (CHHP) in cyclohexane. CHHP is an important intermediate in oxidation of cyclohexane to KA and various processes are known in the art to optimize conversion of CHHP to KA, in order to maximize yield of KA. In addition to K, A, and CHHP, cyclohexane oxidation produces byproducts. In some cases, it has been found that these byproducts interfere with subsequent processing to convert CHHP to KA.
It is known that at least some of the interfering byproducts can be removed by contacting the intermediate stream containing K, A, and CHHP with water or caustic, for example as described in U.S. Pat. No. 3,365,490, which is incorporated herein by reference in its entirety. This patent describes air oxidation of cyclohexane, followed by nitric acid conversion to diacids, such as adipic acid, and processing of byproduct wastestreams. This contacting, or extraction results in a two-phase mixture that, after phase separation, yields a purified cyclohexane stream containing K, A, and CHHP (which can be subjected to known high-yield processes to convert CHHP to KA) and a byproduct water stream. The byproduct water stream (“Water Wash”) contains various mono- and di-acids, hydroxy-acids, and other oxidation byproducts formed during the initial oxidation of cyclohexane.
Regardless of whether a water wash is performed as an intermediate step, the stream containing K, A, and CHHP is further processed by methods well known in the art, to complete conversion of CHHP to K and A. The resulting mixture is then refined, again by methods well known in the art, to recover unconverted cyclohexane for recycle and to obtain purified K and A for subsequent oxidation to adipic acid or conversion to caprolactam. To summarize, the byproduct streams, sometimes referred to herein as “by-product” streams, available from a cyclohexane oxidation process include “Water Wash” (the aqueous stream produced by water extraction of cyclohexane oxidate) and “NVR” (the high-boiling distillation bottoms from KA refining), CAS Registry Number 68411-76-7. Concentration of “Water Wash” by removal of at least some of the water produces a stream known as “COP Acid,” CAS Registry Number 68915-38-8. See also published US patent applications US2004/0054235 describing production of “non-volatile residue”, high-boiling distillation bottoms from distillative recovery of cyclohexane oxidation products cyclohexanol and cyclohexanone, termed “NVR”, having low chromium content, more suitable for combustion, US2012/0064252 and US2012/0101009, incorporated herein by reference, describing processing of NVR, Water Wash, or COP acid through conversion of free acid functional groups to monomeric esters and oligomeric esters, and converting oligomeric esters to monomeric esters.
“Water Wash”, “COP Acid”, and “NVR” are known to contain both mono- and poly-functional materials (functional monomers), mainly with the functional groups comprising acids, peroxides, ketones, alcohols, and esters. Other functional groups such as aldehyde, lactone, and alkene are also known to be present. Multiple functional groups may be combined in a single molecule, such as in a hydroxyacid, for example hydroxycaproic acid or hydroxyvaleric acid. In general, the acid functional group is at one end of a linear hydrocarbyl chain, and the hydroxy group may be present in various positions along the chain. The mono- and poly-functional materials contained within these byproduct streams are primarily aliphatic. Known examples of hydroxyacids include 6-hydroxycaproic acid, 5-hydroxyvaleric acid, 3-hydroxyvaleric acid, and 3-hydroxypropionic acid. Similarly, known examples of simple mono-acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, and caproic acid. Known examples of diacids include succinic acid, glutaric acid, and adipic acid. Known examples of keto-acids include 4-oxo valeric acid (also known as levulinic acid) and 5-oxo caproic acid. Known examples of alcohols include cyclohexanol, 1-propanol, 1-butanol, 1-pentanol, and various diols such as 1,2- 1,3-, and 1,4-cyclohexanediols, various butanediol isomers, and various pentanediol isomers.